The present invention relates to alpha-mosaic ("Teletext"/"Viewdata", etc.) decoders, and more particularly to such decoders that are added on ("set-top") to television receivers.
It is known to transmit pulse signals representing alpha-mosaic characters using eight grey scale levels during the vertical blanking interval of a television signal. Recently, interest has expanded to include the use of color characters. Ideally, a decoder for such characters is built into the television receiver during manufacture. In such case, the decoder provides red (R), green (G), and blue (B) pulse signals directly to the video display circuits in the receiver, thus bypassing the limited-bandwidth chroma channel therein. This allows the display of broad bandwidth (high resolution) characters.
However, there are many receivers in use that do not have built-in decoders. For such receivers, if it is desired to receive and display said characters, an external ("set-top") decoder is required. Set-top decoders apply R, G, and B signals representing the characters to a modulator that modulates an R.F. generator, which generator is set to a frequency corresponding to a locally unused television channel. The modulated R.F. signal is applied to the antenna terminals of the receiver, and in the receiver the signal is demodulated, applied to the luminance and chrominance channels, and then applied to the display circuitry. Thus, the pulse character signals are stretched and have their amplitude reduced by the limited-bandwidth receiver chroma channel. However, the bandwidth of the luminance channel is normally sufficient to pass the signals without appreciable pulse stretching or amplitude reduction.
The legibility and contrast of a character depends inter alia upon the ratio of the amplitude of its color signal to the amplitude of the color signal of the surrounding background. For certain combinations, e.g., yellow character against a white background or blue character against a black background, the legibility is reduced. In the first case, the yellow signal is transmitted through the chroma channel, which reduces its relative amplitude (in IRE units), while the white signal is primarily transmitted through the luminance channel, which does not appreciably reduce its relative amplitude (100 IRE units). The relative amplitude of the yellow signal may be so far reduced with respect to the white signal relative amplitude that it cannot be seen against the white background. The same relative amplitude reduction happens to the blue signal with respect to the relative amplitude of the black signal (0 IRE units), and therefore the blue signal is difficult to see against the black background. In general, the most difficult legibility problem occurs when there is a difference of one grey scale level between a character and its background.
It is therefore an object of the present invention to improve the legibility and contrast of alpha-mosaic characters, and more particularly to achieve such with set-top decoders.